Comet Hale-Bopp Light Curve Update

Mark Kidger
April 25, 1997
mrk@ll.iac.es

We can now sit back and look at the light curve of Comet Hale-Bopp a little
more calmly and assess it in relation to other objects. Here are my conclusions
although I must stress than any comparison with past objects is always
controversial because you cannot rely on old data as being reliable.

Current magnitude: Comet Hale-Bopp is still just slightly brighter than
magnitude 0, although some observers are estimating fainter magnitudes now.
There is, however, no question just by looking at the comet that it is a lot
fainter than it was just a few weeks ago, especially as it is no longer so
obvious in bright twilight. As we will see, its performance has few rivals
over the last half millennium, despite the fact that the best is now over.

The magnitude has faded steadily from what was a very flat maximum around
perihelion, although the comet is actually fading more slowly now than it
brightened before perihelion.

Two all-time cometary records now held by Hale-Bopp:

As far as I can tell from my researches, Comet Hale-Bopp now holds two records,
one of which I would venture will probably never be broken. If there were any
doubt that Comet Hale-Bopp is one of the greatest comet EVER seen and comparable
with the finest objects of recent centuries, I would suggest that that doubt
should now formally be dispelled.

Visibility as a negative magnitude object...

Comet Hale-Bopp appears to have reached magnitude 0 on March 6th (on that day
the same number of observers estimated it to be brighter as estimated a
fainter magnitude). In other words, it has been brighter than magnitude 0
now for no less than 7 weeks and this figure may yet increase to 8. This
appears to beat a long-established record held jointly by two comets. As far
as I can tell from what are definitely unreliable data, the longest ever
periods that a comet has been negative magnitude are:

No other comet appears to have been brighter than magnitude 0 for longer than
2 weeks since the year 1500. Comet De Cheseaux was brighter than magnitude 0
for 7 weeks in total, but could not be observed for an 8 night period in the
middle of this interval, when it rounded perihelion, so I have discounted one
week in the table. The best effort (theoretically) by Comet Halley is about
a week and a half in 837 AD.

Longest ever naked eye visibility

Here I have not tried to be a rigorous in my research but it appears that
Comet Hale-Bopp has already beaten all records by some distance and, as it
will undoubtedly remain visible for as much as six months more, may well set
a mark which will never be approached as light pollution extends itself. As
a comet like Hale-Bopp only comes round every century or so, I hate to think
what light pollution will be like when the next comet as great as Hale-Bopp
finally appears.

The record-holder seems to be a comet which has been compared to Hale-Bopp;
Comet Flaugergues, of 1811, which had a similar orbit to Hale-Bopp as was a
similarly large object. Comet Flaugergues was seen with the naked-eye
reputedly for 9 months. As the first naked-eye observation of Comet Hale-Bopp
was made on May 17th 1996 we are already past 11 months and the comet
promises to remain a naked-eye object until the end of the year, this means
that the final mark may even double Comet Flaugergues's visibility.